With the development of China’s port industry, the crane’s lifting tonnage and operating efficiency are constantly expanding, and the adaptability to the QU crane rail is also put forward higher requirements. In the past, the way of connecting the QU crane rails through the fishplates has limited the running speed of the crane. Especially with the increase of the running speed of the crane, the impact of the fishplate joint on the crane roller is large, which affects the service life of the roller. The connection of the QU crane rail to the fishplate connection to welding is precisely to meet this requirement, in order to achieve the purpose of improving the comprehensive economic benefits of port loading and unloading. Compared with the fishplate connection, the welding method has strong integrity, high reliability, without post-maintenance cost, and can be adapted to the crane’s operating mechanism to the maximum extent, but there are also problems such as high cost, complicated process and long construction period.
The material of the QU120 rail is made of U71Mn. The welding of this QU crane rail made in this material is difficult, mainly because of the following points. The higher carbon content makes the high carbon steel weldment cool after welding, especially in the low temperature season, the quenching performance is sensitive, the martensite structure is generated in the heat affected zone, and the brittle hardening zone is formed, which is easy to crack. At the same time, because the microstructure has effect of stress on the weld metal, this may also affect the crack in the overheated zone. The thermal conductivity of high carbon steel is inferior to that of low carbon steel, resulting in a significant temperature difference between the welded and unheated parts, causing a large stress concentration during welding process, and a tendency to crack when the molten pool cools. When high carbon steel is welded, due to the influence of high temperature, the grain growth speed is fast, part of the carbide is distributed at the grain boundary, the weld seam is weak, and the strength of the welded joint is lowered. During the welding process, the base material is partially melted into the weld metal, which increases the carbon content of the weld metal and is prone to thermal cracking. The liquid metal in the molten pool is exposed to oxygen in the surrounding air for a long time and is easily oxidized. Therefore, strict process measures must be taken to select the appropriate welding parameters to ensure the welding quality.